In recent years, in the field of letterpress printing, a Computer to Plate Technique (CTP Technique) , known as a digital image-forming technique, has been widely used as a general technique. The CTP technique is a method in which information processed on a computer is directly outputted onto a printing plate so that a concave/convex pattern to forma relief is obtained. This technique eliminates the necessity of negative film producing processes, thereby making it possible to reduce costs and time required for forming the negative film.
In the CTP technique, the negative film, conventionally used so as to cover areas that should not be polymerized, is replaced by a mask that is formed and integrated within a printing plate. With respect to the method for obtaining the integration mask, a method has been widely used in which an infra-red ray sensitive layer (a heat sensitive mask layer) that is opaque to chemical rays is formed on a photosensitive resin layer and by evaporating this infra-red ray sensitive layer by using an infrared laser, an image mask is formed (cf. Japanese Patent Application Republication No. 506201/95).
As the heat sensitive mask layer, those layers, made from carbon black serving as an opaque material to radiations and a binder, are generally used. The heat sensitive mask layer is subjected to abrasion by an infrared laser. Therefore, the layer is desirably made thinner from the viewpoint of abrasion efficiency. Here, as the layer is made thinner, influences of the caused wrinkles onto the relief become lesser. However, in order to prevent chemical radiations from being transmitted into a photo-polymerizable layer, the photosensitive mask layer generally needs to have a transmittance optical density (light blocking effect) of 2.0 or more. An ideal heat sensitive mask layer is such a layer as to provide a predetermined transmittance optical density (light blocking effect) or more yet prepared as a thin film.
In general, the optical density (light blocking effect) of the heat sensitive mask layer is defined by the following formula.Optical density=log (100/T)=εcl 
Here, T represents a transmittance (%), c represents a density of an infra-red ray absorbing substance (mol·l−1), l represents a thickness (cm), and ε represents a molecular extinction coefficient (l·mol−1·cm−1). As can be understood by the above-mentioned formula, the optical density can be enhanced by increasing the density of the infrared-ray absorbing material (carbon black) ; however, in this case, the film becomes too brittle, which causes innumerable scratches in working process. The scratched portions fail to shield light, thereby causing unnecessary images to be formed on a relief. Moreover, although the optical density can be increased by making the coat film thickness thicker; however, in this case, high energy is required to cause abrasion, and wrinkles tend to be caused on the relief . Wrinkles on the relief become in particular a serious problem in the case of a water developing plate whose photosensitive resin layer is flexible.
Here, it has been known that the light blocking effect is changed depending on the dispersing state of carbon black even when the carbon black density and the layer thickness are the same. In general, the better the dispersing state, the higher the optical density becomes. Here, the dispersing state of carbon black is highly influenced by a binder resin.
Up to now, as the binder resin for the heat sensitive mask layer, a nylon resin (cf. Japanese Patent Application Laid-Open (JP-A) No. 305030/96) , a polyvinyl alcohol resin (cf. Japanese Patent Application Laid-Open (JP-A) No. 171247/97), a heat decomposable polymer (cf. Japanese Patent Application Laid-Open (JP-A) No. 2002-214792) or a plurality of polymers have been used. In the case where the nylon resin is used, although the durability of the heat sensitive mask layer becomes superior, the dispersing state of carbon black becomes undesirable, causing the necessity of increasing the layer thickness so as to achieve a predetermined optical density. On the other hand, in the case where the polyvinyl alcohol resin is used, the resulting defect is that the durability of the film deteriorates. In the case where the heat decomposable polymer is used as well, it is not possible to obtain a satisfactory abrasion efficiency. Therefore, at present, a heat sensitive mask layer having a high optical density (light blocking effect) with superior durability yet prepared as a thin film has not been achieved.